Fix #3221: bug with importing assembly files

ICSharpCode.ILSpyX/TreeView/SharpTreeNodeCollection.cs

@@ -73,7 +73,10 @@ namespace ICSharpCode.ILSpyX.TreeView
 
 		public SharpTreeNode this[int index] {
 			get {
-				return list[index];
+				if (index >= 0)
+					return list[index];
+				else
+					return null;
 			}
 			set {
 				ThrowOnReentrancy();

ILSpy/Controls/TreeView/SharpTreeView.cs

@@ -561,8 +561,12 @@ namespace ICSharpCode.ILSpy.Controls.TreeView
 			{
 				if (node.IsExpanded && node.Children.Count > 0)
 				{
-					var firstChildItem = ItemContainerGenerator.ContainerFromItem(node.Children[0]) as SharpTreeViewItem;
-					TryAddDropTarget(result, firstChildItem, DropPlace.Before, e);
+					var nodeChild = node.Children[0];
+					if (nodeChild != null)
+					{
+						var firstChildItem = ItemContainerGenerator.ContainerFromItem(node.Children[0]) as SharpTreeViewItem;
+						TryAddDropTarget(result, firstChildItem, DropPlace.Before, e);
+					}
 				}
 				else
 				{